1. Field of the Invention
The present invention relates to a toner used for electrophotographic image formation such as for copiers, electrostatic printing, printers, facsimiles, electrostatic recording and the like, and also relates to an image forming apparatus using the toner, an image forming method using the toner, and a process cartridge using the toner.
2. Description of the Related Art
Conventionally, various methods of electrophotographic image formation have been known in the art. Generally, the surface of a latent electrostatic image bearing member (hereinafter, may be referred to as “photoconductor”, “electrophotographic photoconductor” or “image bearing member”) is charged and the charged surface of the latent electrostatic image bearing member is exposed to form a latent electrostatic image. Next, the latent electrostatic image is developed to form a visible image on the latent electrostatic image bearing member. The visible image is directly transferred onto a recording medium or transferred via an intermediate transfer member onto a recording medium, and the transferred image is fixed by heating and/or pressurizing the transferred image, thereby obtaining a recorded material with an image formed on the recording medium. Then, a residual toner remaining on the surface of the latent electrostatic image bearing member after transfer of the visible image is removed by a known method using a cleaning blade, cleaning brush, cleaning roller or the like.
For a full-color image forming apparatus utilizing such an electrophotographic technique, typically, there are two methods known in the art. One method is called a single method (or single drum method). In the single method, one latent electrostatic image bearing member is mounted inside an image forming apparatus, and four developing units each corresponding to each four colors of cyan, magenta, yellow and black are mounted inside the image forming apparatus. In such a single method, four color visible images are formed on the latent electrostatic image bearing member or a recording medium. The single method enables to share a single charging unit, a single exposing unit, a single transfer unit, a single cleaning unit and the like that are arranged around a latent electrostatic image bearing member, and the single method allows an image forming apparatus to be designed compact at low-production cost, as compared to a tandem method, which will be described hereinafter.
Another method is called a tandem method (or tandem-drum method). In the tandem method, a plurality of latent electrostatic image bearing members are mounted inside an image forming apparatus (see Japanese Patent Application Laid-Open (JP-A) No. 5-341617). Typically, one charging unit, one developing unit, one transfer unit and one cleaning unit are arranged for one latent electrostatic image bearing member, all of them constitute one image forming constitutional element, and a plurality of the image forming constitutional elements (typically, four image forming constitutional elements) are mounted inside an image forming apparatus. In the tandem method, a one-color visible image is formed using one image forming constitutional element, and four-color visible images are sequentially transferred onto a recording medium to thereby form a full-color image. The tandem method enables to produce respective color-visible images by parallel processing, and thus it allows for high-speed image formation. Specifically, the tandem method requires only about one-fourth of the image forming processing time required for the single method and can form an image at a printing speed four-times as high as the printing speed of the single method. Further, the tandem method can virtually increase the durability of respective units such as a latent electrostatic image bearing member in an image forming constitutional element. This is because the tandem method requires just only one operation to perform the above-noted steps using one latent electrostatic image bearing member, in contrast to the single method, in which one latent electrostatic image bearing member goes through four times of respective steps of charging, exposing, developing and transferring to form a full-color image.
However, in the tandem method, it is necessary to arrange a plurality of image forming sections, and thus the method has a disadvantage in that there is a need to increase the size of the main body of an image forming apparatus, resulting in high-production cost.
To solve the aforementioned problem, there is a method of making a latent electrostatic image bearing member have a smaller diameter and down-sized respective units arranged around the latent electrostatic image bearing member, thereby reducing one image forming constitutional element in size. As a result, it is possible to obtain not only an effect of downsizing of the main body of an image forming apparatus but also an effect of reducing material cost, and overall cost-cutting of such an image forming apparatus has proceeded in some degree. With achievement of such compact and down-sized image forming apparatuses, the following new problems are introduced. Respective units to be mounted to an image forming constitutional element must be made to achieve high-performance and to achieve highly increased stability.
Further, recently, demands in the market for energy-saving and high-speed performance in image forming apparatuses, such as printers, copiers and facsimiles, have become strong. To achieve such high-performance in an image forming apparatus, it is important to improve thermal efficiency of a fixing unit.
Generally, in an image forming apparatus, an unfixed toner image is formed on a recording medium such as a recording sheet, printing paper, photosensitive paper and electrostatic printing paper through an image formation process such as electrophotographic recording, electrostatic recording and magnetic recording by an indirect transfer or direct transfer method. As a fixing unit to fix such an unfixed toner image, for example, contact heating methods such as heat roller method, film heating method and electromagnetic induction heating method are widely employed.
The heat roller type fixing unit is basically composed of a pair of rotation rollers of a fixing roller which has a thermal source, such as a halogen lamp, inside thereof to thermally control the temperature to a predetermined value and a pressurizing roller that is pressed against the fixing roller to make contact therewith. A recording medium is inserted into a contact portion (so-called nip portion) of the pair of rotation rollers to convey the recording medium, and an unfixed toner image is fused and fixed on the recording medium by heat and pressure from the fixing roller and the pressurizing roller.
Film heating type fixing units have been proposed, for example, in Japanese Patent Application Laid-Open (JP-A) Nos. 63-313182 and 1-263679. Such a film heating type fixing unit is configured to supply heat via a fixing thin film having heat resistance from a heater which is fixed to and supported with a supporting member by making a recording medium closely contact with the heater via the fixing thin film while sliding the fixing thin film against the heater and moving.
For the heater, for example, a ceramic heater having a heat resistant layer formed on a ceramic substrate made of alumina, aluminum nitride or the like, which has properties such as heat resistance, insulation and excellent thermal conductance, is used. In such a fixing unit, a fixing film which is thin and is of low heat capacity can be used, and it has higher heat transfer efficiency than the above-noted heat roller type fixing unit, enables to shorten warm-up time and allows for quick starting and energy-saving.
For the electromagnetic induction heating type fixing unit, for example, a technique of making a heater having a magnetic metal member electromagnetically induce and generate heat by generating Joule heat using an eddy current generated from an AC magnetic field in the magnetic metal member has been proposed (see Japanese Patent Application Laid-Open (JP-A) No. 8-22206).
In such an electromagnetic induction heating type fixing unit, to wrap around a visible image and to uniformly heat and fuse the visible image, a film having a rubber elastic layer on the surface thereof is placed in between a heater and a recording medium. When the rubber elastic layer is formed with a silicone rubber or the like, the heat responsiveness becomes poor due to its low thermal conductance, resulting in an extremely large temperature difference between the inner surface of the film heated from the heater and the outer surface of the film being contact with a toner. When a toner adhesion amount is large, the belt surface temperature is rapidly lowered, sufficient fixing ability cannot be ensured, and consequently so-called cold offset may occur.
Further, a fixing unit used in an electrophotographic image forming apparatus is required to have toner-releasing property (hereinafter, may be referred to as “offset resistance”) to a heating member. Such offset resistance can be improved by making a releasing agent exist on the surface of the toner, however, when a predetermined toner is used or a used toner is reused, the amount of a releasing agent existing on the toner surface is reduced and the offset resistance of the toner may degrade.
Further, with achievement of an image forming apparatus allowing high-speed performance and energy saving, there is a need to use a toner that is excellent in low-temperature fixing property. In the meanwhile, a toner having offset resistance and storage stability (blocking resistance) conflicting with the low-temperature fixing property is required. To respond to the need, a toner using an aromatic polyester resin is proposed, however, the toner has a shortcoming of being poor in pulverizability in production process. To address the shortcoming, a method is proposed in which a low-molecular weight polyester using an aliphatic alcohol which is excellent in pulverizability as a monomer and a high-molecular weight polyester are blended. (see Japanese Patent Application Laid-Open (JP-A) No. 2002-287427). However, a low-molecular weight polyester using an aliphatic alcohol has a low glass transition temperature because of its structure, the storage stability of the toner degrades, and thus it is difficult to keep both offset resistance and storage stability at a high level. Furthermore, with speeding up of developing in recent years, a toner is required to have a high-charge rising property, however, the toner does not have sufficient charge rising property.
To improve charge rising property of a toner, adding a charge controlling agent in a toner is the most common method. For typical charge controlling agents, chelate compounds containing salicylic acid, oxysalicylic acid or the like as a ligand are exemplified. Metal complex salts of such salicylic acid derivatives are proposed in Japanese Patent Application Laid-Open (JP-A) Nos. 62-145255, 55-42752 and the like, however, in these proposals, the controlling agents respectively contain heavy metal such as Cr and Co, and accordingly it is unfavorable to use them in terms of environmental safety.
Then, in consideration of environmental safety, there have been a number of salicylic acid derivatives proposed which have Fe as a central metal and contains no heavy metals such as Cr and Co therein. For example, Japanese Patent Application Laid-Open (JP-A) No. 1-309072 discloses an effect of limiting controlling agents to be used to metal complexes of salicylic acid derivatives having a carboxyl group or a sulfoxylic group as a substituent group. Japanese Patent Application Laid-Open (JP-A) No. 9-325520 discloses an effect of using only a combination of specific resins and a combination of iron complexes of salicylic acid as ligands. Further, Japanese Patent Application Laid-Open (JP-A) Nos. 7-230188 and 10-10785 respectively disclose an effect of using a combination of iron complexes of specific resins and oxycarboxylic acid as ligands. However, the proposed techniques exert their effects only when limiting it to a combination of a specific resin and an iron complex of salicylic acid derivative.
Further, Japanese Patent Application Laid-Open (JP-A) No. 2001-343787 discloses an effect of using a combination of a non-linear polyester resin which specifies a hydroxy value and a metal complex of salicylic acid derivative, however, when using a polyester resin in which an aliphatic alcohol that is not particularly limited is used as a monomer, the charge amount distribution becomes wide to cause a variation in charge amount among toner particles. Furthermore, because of the structure of the monomer, the mechanical strength of the toner is weak, and the surface of the toner deteriorates by being stirred and shared in a developing device, and the charge amount of toner is significantly reduced with stirring time, which adversely affects quality of image, consequently.
Accordingly, the present situation is that it is desired to immediately provide a toner which is excellent in all the properties of low-temperature fixing property, offset resistance, storage stability, charge rising property, charge stability with time and pulverizability and allows for forming high-quality images over a long period of time, an image forming apparatus using the toner, an image forming method using toner as well as a process cartridge using the toner.